Broadband metamaterial absorber with optically controlled switching for tunable electromagnetic absorption

The laser emission of transverse electric (TE) and transverse magnetic (TM) polarization, in alternating successive spectral ranges, that can occur when external grating diode lasers are wavelength tuned, is theoretically and experimentally investigated. The modulated threshold gain curves of the external cavity for the two polarizations can be made to intersect with each other if a grating is used in such a way as to practically eliminate the difference between the laser diode losses and confinement factors for TE and TM polarization. The result is a sequence of wavelength ranges where the threshold gain is alternately lower for the TE resp. TM polarization. Within each range, a wavelength tunable single mode laser emission is obtained. The phenomenon is described, taking into account the characteristics of the key components of the external cavity, i.e., laser diode structure, antireflective (AR) coating, and grating efficiency. Design tools are proposed for the characteristics of the components necessary for the development of such an external cavity. The theoretical and design concepts are experimentally confirmed in a number of external cavities, using diode lasers emitting around 1.5 /spl mu/m, with different types of antireflective coatings, as well as different grating configurations. >

In this article, we presented our proposal for a metamaterial perfect absorber (MPA) operating in the near-infrared band. Our MPA comprised a trapezoidal silver layer, a substrate layer, and an intermediate alumina layer. The transmission characteristics of MPA were analyzed by the three-dimensional (3D) finite difference time domain method. The simulation results were analyzed by equivalent circuit theory. The results show that the MPA achieved 100% absorption of transverse magnetic (TM) polarization and transverse electric (TE) polarization with resonance wavelength λ = 1492.5 nm. By varying the length of the bottom side of the trapezoid, the MPA’s resonance wavelength could be effectively tuned, and the absorption was maintained at above 99%. With the incidence angle of 0°–55°, the absorption remains above 90% at TE polarization and above 80% at 55°–70°. The absorption at the TM polarization remains above 99% when the incident angle is 0°–70°. A wide angle of incidence and good absorption were achieved. The sensitivity (S) and figure of merit were 228 nm/refractive index unit and 1378 RIU−1, respectively. The proposed absorber with excellent absorption in the range of infrared spectra has a promising application potential in fields such as energy harvesting and infrared sensors.

A wide-angle, wideband, and polarization-insensitive metamaterial (MM) absorber was studied based on the theoretical concept of uniaxial perfect matching layer (UPML) in the terahertz range. The MM absorber was designed as a multi-layered anisotropic array structure consisting of a conductive VIA, a bi-layered slot-FSS, and a split-ring resonator (SRR) separated by porous silica spacers. Each component is optimized to approach the required macroscopic uniaxial property for satisfying the reflection-less boundary condition. The SRRs and VIAs were found to play an important role in maintaining large absorption when the angle of oblique incidence increases under transverse electric (TE) and transverse magnetic (TM) polarization, respectively. The absorption achieves 90% in the frequency regime of 0.9 to 10.5 THz, corresponding to a bandwidth of 168% to the central frequency, and retains such high performance up to 60° oblique incidence for both TE and TM waves.

In this paper, a broadband metamaterial (MM) absorber is presented for X-band applications. A novel eight-resistive-arm (ERA) cell is proposed as an MM unit cell to achieve both broadband absorption and wide incidence angles. The proposed ERA cell is designed using equivalent circuit model and full-wave analysis in order to achieve an absorption ratio higher than 90% in the range of 8.2–13.4 GHz. The experimental results indicate that the absorptivity was greater than 90% in the range of 8–13 GHz for all polarization angles under normal incidence. Under oblique incidence, the measured absorptivity was greater than 90% in the range of 8.2–12.2 GHz up to 60° and in the range of 9.2–12 GHz up to 65° in the transverse electric (TE) mode. In the transverse magnetic (TM) mode, the measured absorptivity was higher than 90% in the range of 9.5–12.4 GHz when the incidence angle was varied from 0° to 60° and remaining a 90% absorption bandwidth in the range of 10–12 GHz up to 65°. Compared to other broadband MM absorbers, the proposed MM absorber exhibited the widest incidence angles in both TE and TM modes.

We investigate dynamic effective anisotropy in photonic crystals (PCs) through a combination of an effective medium theory, which is a high-frequency homogenization (HFH) method explicitly developed to operate for short waves, as well as through numerical simulations and microwave experiments. The HFH yields accurate predictions of the effective anisotropic properties of periodic structures when the wavelength is of comparable order to the pitch of the array; specifically, we investigate a square array of pitch 2 cm consisting of dielectric rods of radius 0.5 cm and refractive index n=6√ within an air matrix. This behaves as an effective medium, with strong artificial anisotropy, at a frequency corresponding to a flat band emerging from a Dirac-like point in transverse magnetic (TM) polarization. At this frequency, highly directive emission is predicted for an electric source placed inside this PC, and this artificial anisotropy can be shown to coincide with a change of character of the underlying effective equation from isotropic to unidirective, with coefficients of markedly different magnitudes appearing in the effective equation tensor. In transverse electric (TE) polarization, we note a second radical change of character of the underlying effective equation, this time from elliptic to hyperbolic, near a frequency at which a saddle point occurs in the corresponding dispersion curves. Delicate microwave experiments are performed in both polarizations for such a PC consisting of 80 rods, and we demonstrate that a directive emission in the form of a + (respectively, an X) is indeed seen experimentally at the predicted frequency 9.5 GHz in TM polarization (respectively, 5.9 GHz in TE polarization). These are clearly dynamic effects since in the quasistatic regime the PC just behaves as an isotropic medium.

An ultra-thin narrow-band metamaterial absorber (MA) based on a periodic array of circular metal patch is numerically proposed at terahertz frequencies. The MA could exhibit an absorption peak at 5.91 THz with the absorptivity of 99.9%. The surface current distributions and distributions of the z-component electric field indicate that high absorption is due to the strong magnetic resonance between the two metallic layers and prefect impedance-matched to the free space at the resonance frequency. By simply assembling a diagonal arrangement of several circular patches with slightly different geometric parameters into a unit cell, the bandwidth of strong absorption (more than 90%) can be effectively improved. Finally, the bandwidth of strong absorption for the four-patch MA achieves 0.98 THz and the corresponding full width at half maximum (FWHM) increases to 1.49 THz. Moreover, the thickness of the four-patch MA is only about 22 times smaller than the central wavelength and it could operate well at wide angles of incidence for both transverse electric (TE) and transverse magnetic (TM) waves.

Ultra-broadband terahertz metamaterial absorber using a simple design method

3D simulation of NSOM operation by a nanosphere as a microscope probe using Boundary Element Method

In this work, a single-band metamaterial absorber (MA) based on a three dimensional (3D) resonant structure is presented. The unit cell is composed of a standing gear-shaped resonator, which is embedded in the dielectric substrate. The proposed 3D MA is ultrathin with a total thickness of 2.3 mm, corresponding 0.077λ0 at its center frequency. The simulation results demonstrate a high absorption peak at 10.1 GHz with absorptivity of 99.9%. The proposed 3D MA is insensitive to the polarization of the incident wave due to its rotationally symmetric structure. Moreover, the proposed 3D MA exhibits a wide-incident-angle stability, as absorptivity of more than 85% can be achieved for both TE and TM incidences with incident angle up to 60°. Most importantly, multiband electromagnetic wave absorption of the stereo MA can be enabled by adjusting the structural parameters of the standing gear. The proposed structure is compatible with 3D printing technology and has potential applications in electromagnetic shielding.

The low light extraction efficiency (LEE) is one of the major factors hindering the application of AlGaN based deep ultraviolet (DUV) light-emitting diodes (LEDs). Here we investigate the LEE of AlGaN based nanowire (NW) DUV LEDs emitting at 275 nm for bare NW, NW integrated with aluminum (Al) bowtie antenna array, and NW with passivation layer under transverse-electric (TE) and transverse-magnetic (TM) polarization. It is observed that by integrating plasmonic Al bowtie antenna array with AlGaN based NW, the LEE up to 83% and 74% can be achieved under TE and TM polarization. In addition, the effect of the three different passivation layer SiO2, SiNx and AlN on the LEE of AlGaN based NW is also analysed, the results suggests that SiO2, which has smaller refractive index than NW core, could extract more photons from the NW and lead to large enhancement of LEE. For SiNx and AlN passivation layer, which has refractive index similar to the NW core, have strong coupling with the NW core, when the thickness of passivation layer satisfy resonance coupling conditions, the LEE could be achieved more than 80% for both TE and TM polarization. These integrated NW/antenna array and NW with passivation layer system can provide guidelines for designing other nano-photonic devices.

We present an analysis of the band structure, the equifrequency surfaces, and the density of states (DOS) for the transverse magnetic (TM) polarization mode of the dielectric superlattice, modeled by means of Dirac-delta functions. This complements a recent article [Phys. Rev E 59, 3624 (1999)] that analyzes the case of transverse electric (TE) polarization. Unfortunately, for this simple model, there is no manifestation of the Brewster effect in the band structure for the TM modes. For large values of the frequency or the grating strength, the equifrequency surfaces essentially degenerate into a set of concentric, hollow, and narrow cylinders centered on the superlattice axis. The DOS is enhanced relative to free space for any frequency and it exhibits discontinuities in the slope at the band edges. These results are relevant to the spontaneous emission by an atom or to dipole radiation in one-dimensional periodic structures. The differences between TE and TM modes are discussed. We take the opportunity to correct an error in the DOS calculation for TE polarization in the article referred above.

Nowadays, with the extensive use of mobile phones, the Electromagnetic (EM) radiation penetration from Radio Frequencies (RFs), particularly into the human head, is an issue that needs resolving. Some serious biological hazards occur inside the human body due to RF radiation accumulation. The RF radiation can be minimized by embodying shielding and coating materials on the front side of the mobile handset. The novelty of the proposed work is the use of mathematical analysis in calculating the Specific Absorption Rate (SAR) absorbed by planar four-layer adult and child head models when exposed to mobile smartphone RF radiation. The variation of SAR with the Angle of Incident (AoI) of the EM wave considers Transverse Electric (TE) and Transverse Magnetic (TM) Polarization. The SAR absorption alteration with the AoI of the EM wave is calculated with the help of the shielding effectiveness parameter of the external Polyethylene Terephthalate (PET) shield coated with conductive copper (Cu) mesh, forming a laminated shield using the methodology of the transmission line method. Furthermore, the SAR variation with AoI for both human head models is calculated theoretically at Sub-6 GHz mobile frequencies of 4.5GHz and 3.6GHz. SAR of 7.41e-12 W/kg and 4.41e-11 W/kg is achieved theoretically for adult and child head models respectively, at 89° TE polarization at 4.5GHz.

Polarization-sensitive terahertz reflective multi-channel beam separation by cascaded configuration

Frequency Selective Surfaces (FSSs) are engineered periodic structures that function as spatial filters, enabling the selective transmission, reflection, or rejection of electromagnetic (EM) waves. These structures are widely utilized in advanced RF and microwave systems for applications such as filtering, electromagnetic shielding, and stealth technology. In this manuscript, we propose an angularly stable and polarization-insensitive band-stop Frequency Selective Surface (FSS) optimized for applications in the lower Ka-band. The proposed FSS is constructed from a periodic two-dimensional array of compact unit cells, each with dimensions of <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$5.5 ~\text{mm} \times 5.5 ~\text{mm}$</tex>. This compact configuration supports seamless integration into high-density millimeter-wave (mmWave) systems. Each unit cell incorporates a concentric dual-ring geometry embedded with orthogonally intersecting cross-slots. This unique configuration enhances the EM wave rejection capability of the surface by inducing multiple resonances. As a result, the FSS achieves a wideband stopband response ranging from 20 GHz to 28.5 GHz, corresponding to a notable fractional bandwidth (FBW) of approximately 34%. The dual-ring structure promotes resonance coupling, while the cross-slot inclusions facilitate effective suppression of incident waves, regardless of polarization orientation. The concentric design further contributes to polarization insensitivity, ensuring consistent stopband performance for both vertically and horizontally polarized waves. In addition, the symmetrical layout enhances angular stability, a critical requirement for real-world deployment. To verify the robustness of the design, full-wave electromagnetic simulations were conducted under varying angles of incidence for both Transverse Electric (TE) and Transverse Magnetic (TM) polarizations. The results reveal negligible shifts in the resonance frequency and stable stopband attenuation across angles up to 60°, confirming the angular resilience of the structure. With its compact form factor, wideband rejection characteristics, and strong angular and polarization stability, the proposed FSS is highly suitable for advanced mmWave applications, including EM interference suppression, co-site interference mitigation, and radar cross-section (RCS) reduction in defense, aerospace, and next-generation communication systems.

Tunable meta-devices trigger immense interest in many applications like biosensing, thermal camouflage, radiative cooling, thermal emitting, infrared imaging, and energy harvesting. This paper presents a polarization-independent broadband tunable absorbing and reflecting meta-device using a phase change material Germanium-Antimony-Tellurium (GST). The proposed design manifests high absorptivity >80% over a broadband range of wavelength spanning the mid-IR regime. Furthermore, by changing the phase of GST material from crystalline to amorphous with the help of external stimuli, the absorbance of the device was reduced to almost zero in the range from 4 μm to 12 μm. To previse the angular constancy of the meta-absorber, the absorptivity was scrutinized under different incidence angles in both transverse magnetic (TM) and transverse electric (TE) polarizations which manifested the robustness of the proposed meta-absorber. As a result, the proposed absorber has tunable broadband absorbance and angular stability, which proclaim great potential in applications such as thermal camouflage, radiation detection, thermal emitter, and stealth technology, etc.